1. Field of the Invention
The present invention relates to a stencil for depositing solder bumps on a substrate, such as a circuit substrate. More particularly, embodiments of the present invention provide for a method of forming a polymeric stencil, and further provide for a polymer stencil, which may be employed for solder-paste printing or for depositing solder on a conductive region of a substrate.
2. Description of the Prior Art
Several solder containing compositions are known and used in the electronics industry. These solder containing compositions usually comprise a powdered metal alloy, fluxing agents, binders, and solvents for the binders. The most prevalent solder containing compositions comprise lead-tin (Pbxe2x80x94Sn) alloys and a fluxing agent. The fluxing agent is used to remove oxides that naturally form on the surface of the alloy powder, and the binder is used to hold the fluxing agent to the solder powder and/or provide a controlled amount of slump for those compositions which are intended to be screen printed. When these solder containing compositions are heated upon reflow, the solvents boil off as a vapor, and the binders and fluxing agents decompose, generating both gaseous byproducts and non-volatile residue. These solder containing compositions are often used to electrically connect the terminals of electrical components and integrated-circuit (IC) chips to printed circuit boards.
Another material used to make such connections are metal-filled epoxy compositions. Copper or silver particles, usually less than 50 xcexcm in size, are used with an epoxy resin and curing agent. The particles usually comprise a high weight percentage of the composition, usually greater than 75% by weight. Upon heating, the epoxy cures to immobilize the metal particles in an electrical network which makes the desired electrical connection. Epoxy resins can include branched polymers having two or more epoxide functional groups. The curing agent such as an anhydride reacts with the epoxide group to form a polymer network. The curing rate of an epoxy can be increased by increasing the number of available reactive epoxy functional groups per resin molecule. One way of characterizing the number of epoxy groups per resin molecule is to measure the number of epoxy groups per weight of resin. This can be characterized as the epoxy xe2x80x9cequivalent weightxe2x80x9d of the resin. Epoxy functional resins having relatively higher rates of cure usually have equivalent weights of less than 200.
Other approaches for connecting electrical components to circuit boards include the so-called xe2x80x9canisotropic conductive filmxe2x80x9d (ACF) and the xe2x80x9canisotropic conductive materialxe2x80x9d (ACM). These approaches are similar in that they both use large spherical or cylindrical conductive bodies which are distributed in a thermosetting or thermoplastic polymer. They are different in that the ACF film is preformed in the form of a bonding sheet while the ACM material is a spreadable liquid. The conductive bodies may comprise metal or metal-coated polymeric materials, which are resilient and elastic. To use the ACM or ACF, the ACM or ACF is placed between an electrical terminal and a corresponding pad and is heated with the terminal being pressed against the pad. The terminal makes electrical contact to the pad through at least one conductive body, with excess polymer being squeezed away from the top and bottom of the body by the applied pressure. Upon heating, the polymer cures and changes from a liquid to a solid. Upon cooling, the polymer shrinks in the vertical direction, and thereby applies a contractive force between the terminal and pad, which in turn maintains pressure on the conductive bodies between the terminal and pad. The term xe2x80x9canisotropicxe2x80x9d arises because the conductive bodies only conduct electrical current vertically between the terminal and pad, rather than in all directions, as would be the case with a metal-filled epoxy. The electrical connections provided by metal-filled epoxies, ACFs, and ACMs usually have higher electrical resistances than those provided by alloy solders (which, by nature, make metallurgic bonds to the terminal and pad). ACFs and ACMs are also known in the art to have long term reliability problems.
In conventional fine pitch bumping processes (i.e., solder bumping on silicon for flip chips), solder deposited on a semiconductor substrate is reflowed after the stencil which is used to deposit the solder is separated from the semiconductor substrate. However, during the separation of the stencil from the semiconductor substrate, the solder can stick to the side walls of apertures in the stencil. This problem can be mitigated by reflowing xe2x80x9cin-situxe2x80x9d the solder when the stencil is still supported by and is adjacent to the semiconductor substrate. However, if the coefficient of thermal expansion of the stencil is substantially higher than the coefficient of thermal expansion of the semiconductor substrate (or other device), the thermal mismatch between the stencil and the semiconductor substrate may cause the formed solder bump to be displaced during the reflow process. Therefore, what is needed and what has been invented inter alia is a polymeric stencil, and method for making same. By using a low coefficient of thermal expansion polymeric stencil to deposit solder, solder is less likely to adhere to the stencil and is also less likely to be displaced during the reflow process by the thermal expansion of the stencil. Embodiments of the polymeric stencil of the present invention also include a generally wrinkle-free structure.
The present invention is motivated by a desire of the inventors to develop a multilayer lamination process in which several circuitized layers are laminated together to form a desired multi-layer circuit substrate structure and in which electrical connections between circuitized layers are formed simultaneously with the lamination process. Because the layers are being laminated at the same time, and because one desires to make the electrical connections between layers, the heat and pressure applied to conventional solder containing compositions can generate significant amounts of gaseous byproducts and non-volatile residue. The gaseous byproducts can leave bubbles between the layers of the formed multi-layer circuit substrate and can potentially contribute to the delamination or failure of the substrate.
Metal-filled epoxies, ACFs and ACMs can instead be used to connect circuit layers. However, these joining materials have reliability issues, particularly if the resulting board underwent long periods of high thermal cycling (i.e., large swings in temperature over long periods of time) and/or humidity exposure. Moreover, metal-filled epoxies, ACFs and ACMs can have relatively high resistivities.
Embodiments of the invention are directed to the need of finding an electrical joining material which can be used in the above multi-layer lamination process and which can provide electrical connections which have higher reliability during thermal cycling, and lower resistance than metal-fill epoxies, ACFs, and ACMs. Other embodiments of the invention can be directed to favorable and less complicated methods for forming multi-layer circuit substrates.
The present invention encompasses conductive compositions which are capable of forming metallurgical bonds to metal terminals and metal pads and do not generate significant amounts of gaseous byproducts or non-volatile residue when processed. The present invention also encompasses articles and methods using the conductive composition.
One embodiment of the invention can be directed to a conductive composition comprising conductive particles in an amount of at least about 75 wt. % based on the weight of the composition. At least 50% by weight of the conductive particles can have melting points of less than about 400xc2x0 C. The composition may also include a carrier including an epoxy-functional resin in an amount of at least about 50 wt. % based on the weight of the carrier. The epoxy functional resin can have a viscosity of less than about 1000 centipoise at 25xc2x0 C., preferably less than about 600 centipoise at 25xc2x0 C. The carrier can also include a fluxing agent in an amount of at least about 0.1 wt % based on the weight of the carrier. The carrier may optionally include other materials.
Another embodiment of the invention can be directed to a method for forming a multi-layer circuit substrate. In particular, embodiments of the invention can be directed to the formation of a multi-layer circuit substrate having a reliable z-connection between conducting surfaces using the inventive conductive composition. A xe2x80x9cz-connectionxe2x80x9d can be a generally vertical electrical connection between conducting layers in substrates or multi-chip modules.
The present invention provides a method for depositing solder on a conductive region of a substrate comprising providing a substrate having a coefficient of thermal expansion; providing a polymeric stencil having a stencil aperture and a coefficient of thermal expansion which is approximately equal to the coefficient of thermal expansion of the substrate; and disposing the polymeric stencil on the substrate such that the stencil aperture is generally aligned with a conductive region on the substrate. The method for depositing solder also comprises depositing the solder-paste through the stencil aperture and on the conductive region; reflowing the solder-paste while the polymeric stencil remains disposed on the substrate; and removing the polymeric stencil from the substrate essentially without any solder-paste being removed with the polymeric stencil. The stencil aperture comprises a diameter approximating a value ranging from about 0.10 mm to about 0.20 mm. The present invention also provides a stencil for use in a fine pitch bumping process comprising a releasable polymeric sheet (e.g., a polymeric sheet comprising a polyimide polymer and/or a tetrafluoroethylene fluorocarbon polymer and/or a fluorinated ethylene-propylene polymer) having a coefficient of thermal expansion of less than about 4 ppm/xc2x0C. and a structure defining a pair of apertures spaced from each other at a distance of less than about 0.20 mm. Each aperture comprises a diameter approximating a value ranging from about 0.10 mm to about 0.20 mm (e.g., about 0.15 mm). The apertures are preferably spaced from each other at a distance less than bout 0.15 mm. Preferably, each aperture includes a tapering wall and is surrounded by a compressed, substantially wrinkle-free polymeric structure.
The present invention further provides a method of forming a polymeric stencil for solder-paste comprising forming in a polymeric sheet a plurality of apertures, preferably tapered apertures, having wrinkles in the polymeric sheet in proximity to the plurality of apertures; compressing opposing surfaces of the apertured polymeric sheet toward each other; heating the compressed apertured polymeric sheet; and rapidly cooling the compressed apertured polymeric sheet to remove wrinkles in proximity to the apertures. The apertures include a tapering wall having an angle with a horizontal plane ranging from about 40 degrees to about 80 degrees, preferably ranging from about 55 degrees to about 75 degrees. Forming of the plurality of apertures may include heating the polymeric sheet in proximity to the apertures. The present invention also further provides a method of solder-paste printing a substrate comprising providing a polymeric sheet having an aperture surrounded by a compressed, substantially wrinkle-free polymeric structure; placing the polymeric sheet on a substrate; and disposing a solder-paste through the aperture and on the substrate to solder-paste print the substrate.
These provisions together with the various ancillary provisions and features which will become apparent to those skilled in the art as the following description proceeds, are attained by the methods and stencils of the present invention, preferred embodiments thereof being shown with reference to the accompanying drawings, by way of example only, wherein: